FIG. 11A shows a schematic plane view of an inside of a known optical scanning device such as an electronic copier, a laser beam printer, or a laser facsimile equipment. FIG. 11B shows an oblique drawing of a scanning lens being used in the optical scanning device.
As shown in FIG. 11A, the optical scanning device is provided with a light source unit 1, a rotary polygon mirror 2, and a scanning lens 4.
Light source unit 1 includes elements such as a semiconductor laser or a collimate lens. Rotary polygon mirror 2 as a scanning. means scans a parallel laser beam L1 emitted from light source unit 1. The scanned laser beam L1 passes through a reflection mirror. Scanning lens 4 focuses the parallel beam L1 to form an image on a photosensitive body on a rotary drum (not shown).
Rotary polygon mirror 2 and scanning lens 4 are housed inside an optical box 5, and light source unit 1 is secured to a sidewall of optical box 5.
After assembling the parts including light source unit 1, rotary polygon mirror 2 and so forth inside optical box 5, an upper-part opening of optical box 5 is capped with a lid (not shown).
A window 6 is provided on a sidewall of optical box 5. The light beam L1 scanned by rotary polygon mirror 2 goes through window 6 toward rotary drum (not shown) arranged out of optical box 5.
A collimate lens (not shown) collimates laser beam L1 emitted from a semiconductor laser of light source unit 1. A cylindrical lens la converges the collimated laser beam L1 into a linear light beam on a reflecting surface of rotary polygon mirror 2. Reflection mirror 3 reflects laser beam L1 from rotary polygon mirror 2 and scanning lens 4 to rotary drum (not shown). The reflected laser beam L1 go through window 6 of optical box 5.
Consequently, the light beam L1 is focused into images on the photosensitive body on the rotary drum. The light beams form electrostatic latent images as the main-scanning rotation by rotary polygon mirror 2 and the sub-scanning rotation by the rotary drum are carried out.
Scanning lens 4 corrects distortions of point images which are formed on the photosensitive body. It is because scanning lens 4 performs as a so-called f-theta lens. Scanning lens 4 is an axially asymmetrical aspherical lens and is integrally molded with plastic.
As shown in FIG. 11B, a lens frame 4x is arranged on the bottom portion of scanning lens 4. Lens frame 4x projects from both sidewalls of scanning lens 4 respectively, and another lens frame 4y is arranged on the upper portion of scanning lens 4. Lens frame 4y projects from both sidewalls of scanning lens 4 respectively. A lens surface (effective-surface) 4c of scanning lens 4 is provided between lens frame 4x and lens frame 4y. 
Furthermore, scanning lens 4 has a pair of projecting parts 11, at the center of flanges 4x and 4y in the longitudinal direction of flange 4x and 4y. Flanges 4x and 4y respectively projects in the direction of the optical axis of scanning lens 4. One of projecting parts 11 is held between a pair of positioning members 16a and 16b. Positioning members 16a and 16b projects from the bottom wall of optical box 5, and position scanning lens 4 in the main-scanning direction of scanning lens 4e. 
In addition, a bottom wall 13 of scanning lens 4 is finished with high flatness as a horizontal reference surface for accurate positioning.
Bottom wall 13 of scanning lens 4 contacts with a pair of horizontal pedestal parts (not shown) which project from the bottom wall. Bottom wall 13 and the pair of horizontal pedestal parts positions scanning lens 4 in the sub-scanning direction of scanning lens 4.
A pair of positioning ribs 14 and 15 are formed on both the side edges of scanning lens 4 respectively. Positioning ribs 14 and 15 have vertical reference surface parts which are perpendicular to the optical-axis direction of scanning lens 4. Positioning ribs 14 and 15 contact with a pair of vertical surface (not shown) provided on a sidewall or a partition wall of optical box 5. Positioning ribs 14 and 15 position scanning lens 4 in the optical-axis direction of scanning lens 4.
When scanning lens 4 is assembled with in optical box 5, one of projecting parts 11 is hold between positioning members 16a and 16b which are provided at the bottom of optical box 5. The horizontal reference surface provided at bottom wall 13 of scanning lens 4 is contacted with the horizontal pedestal parts of optical box 5. Positioning ribs 14 and 15 are respectively contacted with the vertical surfaces parts of optical box 5. Thus, scanning lens 4 is positioned in the main-scanning direction, in the sub-scanning direction, and in the optical-axis direction. After the positioning, scanning lens 4 is fixed on the bottom wall of optical box 5 with conventional ways, such as using adhesives or springs.
It is important to position scanning lens 4 in the main-scanning direction, the sub-scanning direction, and the optical-axis direction against rotary polygon mirror 4 in order to provide good images. The scanning lens mentioned above is described in Japanese Patent Publication (Kokai) No. 09-329755.
FIG. 12A and FIG. 12B respectively show a plane and front view of another type of known scanning lens. As shown in FIG. 12A, a positioning protrusion 11a may be provided on only single sidewall of scanning lens 4a at the center of scanning lens 4a. End parts 25a, which are provided at both ends in the main-scanning direction of scanning lens 4a may be positioned by pins (not shown), for example, provided on an optical box 5. The pins serves to prevent end parts 25a from moving to position scanning lens 4a in the predetermined direction.
Further, FIG. 13A is a cross section of another type of known lens, and FIG. 13B shows a plane view of the lens.
A lens flame 19 is provided to surround lens part 18. Projection 11a provided at the center in the scanning direction of lens 4b positions a scanning lens 4b. Moreover, lens flame 19 prevents lens surface from being damaged in transporting, and from varying the optical characteristics owing to water vapor absorption from the sidewalls of lens 4b. 
It is difficult for the scanning lens shown in FIGS. 12A and 12B to protect a lens surface in transporting and to prevent optical characteristics from varying owing to water vapor absorption.
The scanning lens shown in FIGS. 13A and 13B causes a problem, which is described below.
Firstly, the thickness of lens part 18 is not uniform in the scanning direction, while the thickness of lens frame 19 is uniform in the scanning direction. Thus, in comparing the cross sectional area ratio of lens part 18 with that of lens frame 19, the cross sectional area of lens part 18 relative to lens flame 19 at the center in the scanning direction is larger than that at the end parts in the scanning direction.
FIG. 15 describes resin fluidity in resin molding depending on cross sectional area as mentioned above.
When resin is inserted through a gate G, and pressure is applied to the resin, the tip portion of inserted resin flow ununiformly in the scanning direction. The tip shape of the inserted resin is quite different at the end part from that at the center part.
For example, at the end part in the scanning direction, the resin in lens frame 19 flows faster than the resin in lens part 18, while at the center part in the scanning direction where lens part 18 thickens, the resin flows faster in lens part 18 than in lens flame 19.
Consequently, ununiform fluidity of resin tip produces flow marks FM or rapid change of internal strain, etc., which deteriorate optical characteristics of lenses.
Moreover, providing a projection 11a shown in FIG. 12A and FIG. 13B widens and heightens a lens.
Further, scanning lens 4 shown in FIG. 11 has lens frames 4x and 4y whose projection amount from the lens surface partially vary relatively much in the main-scanning direction as shown in FIG. 11A. Consequently, flow marks or rapid change of internal strain, etc. may occur which deteriorate optical characteristics of lenses.